1. Field of the Invention:
The present invention relates to a method of producing polyphenylene ethers protected against molecular weight degradation and reuse of the copper salts employed in the coupling reaction.
2. Description of the Prior Art:
Polyphenylene ethers and various methods for producing these are known. For example, see Buehler, 1978, "Specialty plastics" pub. Akademiverlag, Berlin and U.S. Pat. Nos. 3,306,874 and 3,306,875.
The industrial process based on oxidative coupling of di-ortho-substituted phenols by means of copper-amine complexes in the presence of oxygen or oxygen-containing gas mixtures has proven commercially successful. Three steps are necessary to produce the pulverulent end product with such processes; namely (1) termination of the polycondensation, (2) purification of the polymer, and (3) isolation of the polyphenylene ether.
The polycondensation may be terminated by alternate means: inactivation of the catalyst, removal of the catalyst, or removal of the polymer. These measures are characterized in that, although they terminate the polycondensation, an incomplete purification of the polymer results.
The most important methods of purification of the polymer are the following.
1. Addition of aqueous acids.
2. Addition of alkalis.
3. Addition of copper ions complexing agents.
4. Addition of polar solvents for separating out the polymers.
The addition of aqueous acid solutions is possibly the most frequently used technique for terminating the polycondensation, even though the amount of acid consumed is substantial, and the degree of amine separation or removal is inadequate (see German OS No. 22 46 552, p. 4 lines 1 to 8). An improvement of this technique (German OS No. 21 05 372) consists of using an extraction apparatus, e.g. a countercurrent apparatus with the reaction liquor flowing in one direction and an aqueous acid solution flowing countercurrent to it with the further possibility of using elevated temperature conditions. With this improvement, however, large amounts of alkali metal hydroxides are required to recover the amines.
According to the method described in German OS No. 22 46 552, the polyphenylene ether reaction mixture, containing the copper-amine catalyst and the polymer in an aromatic solvent, is terminated by treatment with CO.sub.2 and water. The quenched reaction mixture is then processed to isolate the product. The degree of separation of the amine from the polymer is inadequate with this process however.
In German OS No. 15 70 683, the addition of a 50% NaOH solution to the polyphenylene ether reaction mixture is described. This technique has not proven economically feasible, however, because it does not interrupt the polymerization reliably (see German OS No. 24 30 130).
Alternatively, the catalyst can be inactivated and the copper separated out, with the aid of chelating agents (see German Pat. No. 1,570,683, col. 15 lines 3-5; and German OS No. 26 40 147). Additional experimental details for this method may be obtained from German OS No. 23 64 319. The basic drawback of all known methods based on chelating agents is however polymer degradation polymer. It has been established (see German OS No. 27 54 887) that polyphenylene ethers decompose more rapidly the higher the temperature to which they are exposed and the longer the time elapsing between the end of the reaction and polymer isolation. For example, the viscosity index lwhich is an indication of the mean molecular weight of the polymer units) decreases typically by more than 0.1 dl/g per hour, often by as much as 0.2 dl/g/hr (German OS No. 27 54 887).
The use of special chelating agents, e.g. polyamines in the process of Ger. OS No. 24 60 323, has not been able to eliminate this decomposition.
It has been proposed to add additional stabilizing agents to the polymer solutions in addition to the chelating agents. Such proposed agents include, e.g., dihydroxybenzenes, reducing agents (see German OS No. 27 54 887 and German Pat. No. 2,616,746) or aromatic amines (see German OS No. 27 55 937). These solutions, however, cannot yield satisfactory results, because the adjuvants must be supplied in fairly large amounts. Additionally, these adjuvants make it much more difficult to isolate the polyphenylene ether product.
The polyphenylene ether condensation can also be terminated and the polymer precipitated out with the aid of alcohols, ketones, or aqueous solutions of alcohols and/or ketones. These are referred to collectively as "antisolvents". With this technique residues of catalyst and phenoquinones remain behind in the solvent, but small amounts of copper are carried along with the polymer. These small amounts of copper have a detrimental effect on the color of the polymer and in addition catalyze polymer decomposition (see German OS No. 26 16 746).
A common drawback of all the precipitation methods (see, e.g., German OSs Nos. 12 65 410, 25 32 477, 25 32 478, 26 55 161, and 27 52 867) is that substantial amounts of solvents must be employed, processed, and redistilled. Accordingly, it has been proposed that the polymer be recovered by a so-called direct isolation technique, e.g. steam stripping, spray drying, or hot water comminution with filtration. However a precondition for the use of these techniques is that residues of the copper catalyst and other impurities be substantially quantitatively removed prior to the direct isolation stage (see German OS No. 24 60 323).
Finally, according to the prior art the polyphenylene ether reaction can be terminated by simultaneous treatment with a dihydroxybenzene or benzoquinone and a mild reducing agent, e.g. hydrazine (see German Pat. No. 2,430,130). It may be seen from this reference that the resulting polymer does not suffer a reduction in molecular weight after one day. If used alone, the reducing agent is ineffective however. Thus, e.g., if hydrazine in a 30-fold molar excess is used to terminate the reaction, the viscosity of the product decreases from 0.61 to 0.44 dl/g after one day (see ibid., Table I, Test 7). However, when the technique described is employed, the polymer does not suffer from a reduction in molecular weight but the added dihydroxybenzenes or benzoquinones become a hindrance, since they are difficulty biodegradable (see German OS No. 27 54 887).
Another fundamental drawback of all known methods of producing polyphenylene ethers is the fact that the catalyst is irretrievably broken down in the reaction work-up. As a rule, complex and costly process steps are needed to refine the precipitated copper residues and convert them back to active catalyst form. Such steps are, e.g., extraction with a chelating agent, precipitation as the sulfide, conversion to the oxide, and dissolution in hydrobromic acid. An alternative possibility, also costly, is to electrolytically separate the copper salt from the termination solutions which are aqueous acid solutions, aqueous alkali solutions, or chelating agent-containing solutions, and then to convert the recovered copper material to reusable copper salts.
Therefore, there continues to exist a strong need for a method producing polyphenylene ethers protected from molecular weight degradation. There also continues to exist an also strong need for a method for producing polyphenylene ethers where the copper catalyst used in the coupling reaction can be easily reused.